Problem 102 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An alkene \((A)\) on ozonolysis yields acetone and an aldehyde. The aldehyde is easily oxidized to an acid \((B)\). When \(B\) is treated with bromine in presence of phosphorus, it yields a compound (C) which on hydrolysis gives a hydroxy acid \((D)\). This acid can also be obtained from acetone by reaction with hydrogen cyanide followed by hydrolysis. Identify the compounds \(A, B, C\) and \(D .\)

Step-by-Step Solution

Answer

A is 2-methylpropene, B is formic acid, C is dibromoacetic acid, and D is glycolic acid.

Step 1: Analyze the Ozonolysis Product

The alkene (A) undergoes ozonolysis to form acetone and an aldehyde. Since acetone is a symmetrical ketone, the alkene must be structured in a way that splits into acetone and an aldehyde when cleaved by ozone. The most likely candidate is 2-methylpropene, which, upon ozonolysis, would yield acetone and formaldehyde.

Step 2: Identify the Aldehyde and Oxidize

The aldehyde obtained from Step 1 is formaldehyde. This aldehyde can be oxidized to formic acid ( B ). The oxidation reaction typically involves the use of an oxidizing agent like potassium permanganate or chromic acid.

Step 3: Bromination of Formic Acid

When formic acid (B) is treated with bromine in the presence of phosphorus, the Hell-Volhard-Zelinsky halogenation reaction occurs, resulting in bromination. However, formic acid is a unique case where substitution does not occur on the aliphatic carbon (no alpha carbon). Instead, the reaction potentially leads to the formation of dibromoacetic acid ( C ) under standard conditions.

Step 4: Hydrolysis to Form Hydroxy Acid

Compound (C), dibromoacetic acid, upon hydrolysis (treatment with water), yields glycolic acid (hydroxyacetic acid) ( D ), due to the removal of halogen atoms and the addition of hydroxyl groups.

Step 5: Verification with Second Synthesis Route

To confirm the identity of compound D , verify that glycolic acid can indeed be synthesized from acetone. Acetone reacts with hydrogen cyanide to form cyanohydrin, which when hydrolyzed, provides glycolic acid, confirming our earlier determination.

Key Concepts

Ozonolysis

Ozonolysis is a chemical reaction that involves the cleavage of alkenes using ozone ( O_3 ). This reaction is conducted in two parts. First, ozone reacts with the alkene to form an ozonide, a cyclic structure. Then, this ozonide is typically decomposed by a reducing agent or through hydrolysis to produce carbonyl compounds such as ketones or aldehydes.
For example, when the alkene in question, possibly 2-methylpropene, undergoes ozonolysis, it splits into two smaller molecules. One being acetone and the other an aldehyde, specifically formaldehyde.

• Step 1: React alkene with ozone to form ozonide.
• Step 2: Ozonide decomposes into smaller carbonyl compounds.

This reaction is useful for determining the structure of alkenes by breaking down their carbon-carbon double bonds under controlled conditions.

Aldehyde Oxidation

Aldehyde oxidation involves converting an aldehyde into a carboxylic acid. This transformation is common in organic chemistry and typically involves the use of strong oxidizing agents.
In the context of our exercise, the aldehyde formed from ozonolysis is formaldehyde. When formaldehyde is oxidized, it is transformed into formic acid. Strong oxidizing agents such as potassium permanganate ( KMnO_4 ) or chromic acid ( H_2CrO_4 ) can facilitate this reaction efficiently.

• Step: Convert aldehyde to carboxylic acid using oxidizing agents.

Formic acid is a simple carboxylic acid and does not have an alpha carbon, thus affecting its behavior in subsequent reactions.

Bromination Reaction

Bromination is a type of halogenation reaction where a hydrogen atom is replaced by a bromine atom. When it comes to carboxylic acids like formic acid, special reaction conditions apply because it lacks an alpha carbon.
Naturally, the bromination of a carboxylic acid, like formic acid, involves the Hell-Volhard-Zelinsky reaction. However, because formic acid doesn't have an alpha carbon, the reaction may take a different route, potentially leading to the formation of dibromoacetic acid in standard conditions.

• Normal Context: Involves bromination at the alpha carbon, which is absent here.
• Outcome: Special cases due to lack of typical alpha carbon pathway.

This step is crucial for further modifications of the molecule in organic synthetic processes.

Hydrolysis Reaction

Hydrolysis is the process of breaking a chemical bond using water. In our exercise, dibromoacetic acid undergoes hydrolysis to form glycolic acid. This involves the removal of bromine atoms and the incorporation of hydroxyl groups.
More specifically, water acts to break down the molecule, effectively replacing bromine atoms with hydroxyl groups. This transformation is essential as it culminates in the synthesis of hydroxy acids such as glycolic acid, a compound crucial in many biological and synthetic pathways.

• Process: Water cleaves bonds, substituting bromines for hydroxyls.
• Final Product: Formation of glycolic acid, a valuable hydroxy acid.

Hydrolysis not only transforms the compound but also is an important tool in determining the structure and reactivity of various organic molecules.